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Register a new userSurface effects on the Mott-Hubbard transition in archetypal V$_2$O$_3$
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We present an experimental and theoretical study exploring surface effects on the evolution of the metal-insulator transition in the model Mott-Hubbard compound Cr-doped V$_2$O$_3$. We find a microscopic domain formation that is clearly affected by the surface crystallographic orientation. Using scanning photoelectron microscopy and X-ray diffraction, we find that surface defects act as nucleation centers for the formation of domains at the temperature-induced isostructural transition and favor the formation of microscopic metallic regions. A density functional theory plus dynamical mean field theory study of different surface terminations shows that the surface reconstruction with excess vanadyl cations leads to doped, and hence more metallic surface states, explaining our experimental observations.
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We have performed sound velocity measurements in (V$_{1-x}$Cr$_x$)$_2$O$_3$ in the vicinity of the critical point of the first order Mott transition line. The pressure sweeps at constant temperature reveal a large dip in the $c_{33}$ compression modulus, this dip sharpens as the critical point is approached. We do not observe signs of criticality on the shear modulus $c_{44}$ which is consistent with a transition governed by a scalar order parameter, in accordance with the DMFT description of the transition. However, the amplitude of the effect is an order of magnitude smaller than the one obtained from DMFT calculations for a single band Hubbard model. We analyze our results using a simple model with the electronic response function obtained from the scaling relations for the conductivity.
V2O3 famously features all four combinations of paramagnetic vs antiferromagnetic, and metallic vs insulating states of matter in response to %-level doping, pressure in the GPa range, and temperature below 300 K. Using time-of-flight neutron spectroscopy combined with density functional theory calculations of magnetic interactions, we have mapped and analyzed the inelastic magnetic neutron scattering cross section over a wide range of energy and momentum transfer in the chromium stabilized antiferromagnetic and paramagnetic insulating phases (AFI & PI). Our results reveal an important magnetic frustration and degeneracy of the PI phase which is relieved by the rhombohedral to monoclinic transition at $T_N=185$ K due to a significant magneto-elastic coupling. This leads to the recognition that magnetic frustration is an inherent property of the paramagnetic phase in $rm (V_{1-x}Cr_x)_2O_3$ and plays a key role in suppressing the magnetic long range ordering temperature and exposing a large phase space for the paramagnetic Mott metal-insulator transition to occur.
Unveiling the physics that governs the intertwining between the nanoscale self-organization and the dynamics of insulator-to-metal transitions (textit{IMT}) is key for controlling on demand the ultrafast switching in strongly correlated materials and nano-devices. A paradigmatic case is the textit{IMT} in V$_2$O$_3$, for which the mechanism that leads to the nucleation and growth of metallic nano-droplets out of the supposedly homogeneous Mott insulating phase is still a mystery. Here, we combine X-ray photoemission electron microscopy and ultrafast non-equilibrium optical spectroscopy to investigate the early stage dynamics of isolated metallic nano-droplets across the textit{IMT} in V$_2$O$_3$ thin films. Our experiments show that the low-temperature monoclinic antiferromagnetic insulating phase is characterized by the spontaneous formation of striped polydomains, with different lattice distortions. The insulating domain boundaries accommodate the birth of metallic nano-droplets, whose non-equilibrium expansion can be triggered by the photo-induced change of the 3$d$-orbital occupation. We address the relation between the spontaneous nanotexture of the Mott insulating phase in V$_2$O$_3$ and the timescale of the metallic seeds growth. We speculate that the photoinduced metallic growth can proceed along a non-thermal pathway in which the monoclinic lattice symmetry of the insulating phase is partially retained.
We present the observation of an isostructural Mott insulator-metal transition in van-der-Waals honeycomb antiferromagnet V$_{0.9}$PS$_3$ through high-pressure x-ray diffraction and transport measurements. The MPX$_3$ family of magnetic van-der-Waals materials (M denotes a first row transition metal and X either S or Se) are currently the subject of broad and intense attention, but the vanadium compounds have until this point not been studied beyond their basic properties. We observe insulating variable-range-hopping type resistivity in V$_{0.9}$PS$_3$, with a gradual increase in effective dimensionality with increasing pressure, followed by a transition to a metallic resistivity temperature dependence between 112 and 124 kbar. The metallic state additionally shows a low-temperature upturn we tentatively attribute to the Kondo Effect. A gradual structural distortion is seen between 26-80 kbar, but no structural change at higher pressures corresponding to the insulator-metal transition. We conclude that the insulator-metal transition occurs in the absence of any distortions to the lattice - an isostructural Mott transition in a new class of two-dimensional material, and in strong contrast to the behavior of the other MPX$_3$ compounds.
The local structure of V$_{2}$O$_{3}$, an archetypal strongly correlated electron system that displays a metal-insulator transition around 160 K, has been investigated via pair distribution function (PDF) analysis of neutron and x-ray total scattering data. The rhombohedral-to-monoclinic structural phase transition manifests as an abrupt change on all length scales in the observed PDF. No monoclinic distortions of the local structure are found above the transition, although coexisting regions of phase-separated rhombohedral and monoclinic symmetry are observed between 150 K and 160 K. This lack of structural fluctuations above the transition contrasts with the known presence of magnetic fluctuations in the high-temperature state, suggesting that the lattice degree of freedom plays a secondary role behind the spin degree of freedom in the transition mechanism.
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